Nowadays, scanning devices are popularly used in daily and official affairs. The so-called scanning devices include, such as, fax machine, printer, scanner and multi function peripheral device. It is generally necessary for users to scan both sides of documents. Usually, a low-level scanning device does not provide such function and they have to turn over the sheet manually when performing a duplex scanning. However, it is highly time consuming and laboring when performing duplex scanning for a lot of documents. Besides, for saving space, the volume of the scanning device should not be too large.
Therefore, the advanced scanning device must have the function for automatically scanning double sides of document and would not occupied too large space. FIGS. 1A to 1D show an operation process of a conventional document feeder. As shown in FIG. 1A, a sheet 10 is fed into the document feeder by a pick-up module 20 and then transported to an eject module 40 via a feed module 30. The leading edge of the sheet 10 pushes the bottom of a guide strip 50 to enter the eject module 40. In the above transporting process, the content of the first face of the sheet 10 is scanned by a scanning module 60 of a scanning device 80. Please refer to FIG. 1B, which shows the situation when the trailing edge of the sheet 10 has passed through a first active roller 31 and a first idle roller 32 of the feed module 30. Under this situation, the sheet 10 is transported by the eject roller module 40, and a second active roller 41 and a second idle roller 42 of the eject roller module 40 have to generate a sufficient nip force to precisely control the movement of the sheet 10.
Please refer to FIG. 1C, which shows the situation when the first face of the sheet 10 had completely passed through the scanning module 60 via the rotation of the feed module 30 and the trailing edge of the sheet 10 had reached the eject module 40. Under this situation, the second active roller 41 of the eject roller module 40 starts to rotate reversely to transport the sheet 10 to the feed module 30 via the guiding of the guide strip 50, therefore, the image of the second face of the sheet 10 will be subsequently scanned by the scanning module 60. Last, referring to FIG. 1D, which shows an overlap in the eject module 40 occurred between the leading edge and the trailing edge of the sheet 10 because of the operation of the feed module 30. Under this situation, the second active roller 41 of the eject roller module 40 starts to operate with a positive rotation to make the leading edge of the sheet 10 exit from the document feeder, in the meantime, the second idle roller 42 still rotates in reverse to transport the trailing edge of the sheet 10 into the feed module 30.
The above-mentioned paragraphs illustrate the process of how the conventional document feeder performing the automatic duplex scanning. In the eject roller module 40 of the conventional document feeder, the nip force between the second active roller 41 and the second idle roller 42 must be large enough to precisely control the motion accuracy of the sheet 10, but as can be seen in FIG. 1D, if the nip force between the second active roller 41 and the second idle roller 42 is too large, the leading edge of the scanned sheet 10 will probably fail to be transported into the nip between the second active roller 41 and the second idle roller 42 and result in a jam condition. On the contrary, as can be seen in FIG. 1B, if the nip force between the second active roller 41 and the second idle roller 42 is insufficient, the second active roller 41 and the second idle roller 42 can not precisely transport the sheet 10 and result in a distorted scanning result.
In order to obviate the defects of the conventional document feeder, it is an object of the present invention to provide a duplex document feeder, in which a sufficient nip force is provided to precisely control the motion accuracy of the sheet. Furthermore, the jam will not happen in the scanning process by the present invention.